Nucleotide sequence and characterization of the Rhodobacter sphaeroides glnB and glnA genes

The glnA gene of Rhodobacter sphaeroides encoding glutamine synthetase (GS) has been cloned and sequenced. Molecular analysis revealed that there is a glnB gene upstream of glnA, in a single glnBA operon. A putative glnAp1-type promoter sequence, a consensus ntrC gene product binding site and a consensus upstream activator sequence were detected upstream of the glnB gene. The deduced amino acid sequences of the GS and GlnB proteins of R. sphaeroides showed strong homology with the same proteins from other Gram-negative bacteria. The sequence of the glnA gene isolated from glutamine auxotroph Gln83 was also determined. The glnA83 mutation was shown to result in premature termination of GS synthesis and formation of a 17 kDa C-truncated GS which could be complemented by a 5′-truncated glnA gene which encodes a 30 kDa N-truncated GS. This phenomenon is characteristic for interallelic complementation.